1. Field of the Invention
The present invention relates generally to tunnel excavating methods, and more particularly to a center-cut blasting method for tunnel excavation utilizing large unloaded blast holes and a circular pre-split, which employs a pre-splitting technique that blasts loaded auxiliary holes alternately arranged along with large unloaded auxiliary holes in a circle and a sub-drilling technique of rooting away blast holes, thereby facilitating a center-cut operation by weakening the binding force of an original rock and easily achieving dual free surfaces, and shortening the period of execution and reducing execution costs.
2. Description of the Prior Art
Recently, in order to excavate tunnels and underground spaces for underground storage facilities, communication or electrical cable tunnels, waterway tunnels and traffic tunnels, blasting for tunnel excavation is frequently performed.
In general, such tunnel blasting is performed in the following three stages.
With reference to FIG. 1, a first stage of the tunnel blasting is the stage of drilling center-cut holes 1, cut spreader holes 2, floor holes 3 and roof holes 4 to predetermined depths, a second stage is the stage of loading the drilled holes 1, 2, 3 and 4 with detonators and explosives, and a third stage is the stage of detonating the detonators using a triggering device. Referring to FIG. 1, the detonators are detonated from the center-cut holes 1 to the outer holes 2, 3 and 4.
The center-cut holes 1 are loaded with delay detonators in such a way that the detonators are bilaterally symmetrically arranged in the center-cut holes 1 in the order of detonation in an upward direction. The cut spreader holes 2, the floor holes 3 and the roof holes 4 are loaded with delay detonators in such a way that the detonators are arranged in the holes 2, 3 and 4 in the progressing order from the center to the outside. In such a state, the tunnel is blasted by detonating the detonators using a triggering device. The detonations of the detonators are sequentially performed in the progressing order; the center-cut holes 1, the cut spreader holes 2, the floor holes 3 and the roof holes 4.
Tunnel blasting is mostly performed using a single free surface, and generally employs center-cut blasting so as to create new free surfaces.
The center-cut blasting creates dual free surfaces by blasting the center portion of the working face, and is an important factor that governs the success of the entire tunnel blasting.
In FIG. 1, reference characters 1xe2x80x2, 2xe2x80x2, 3xe2x80x2 and 4xe2x80x2 designate a center-cut region, a cut spreader region, a floor region and a roof region, respectively.
The xe2x80x9cfree surfacexe2x80x9d denotes the surface of rock in contact with an alien sphere, such as air or water. The free surface considerably affects blasting. That is, a blasting effect is in proportion to the number of free surfaces and the degree of proximity of loaded positions to the free surfaces. The reason for this is that resistance is weak in a free surface side and thus blasting energy generates heavy stresses in the free surface side. A xe2x80x9cburdenxe2x80x9d denotes the shortest distance from a free surface to the center of an explosive.
The xe2x80x9ccenter-cut holexe2x80x9d denotes blast holes within a center-cut region of 1.5 to 2.5 m by 1.5 to 2.5 m. A xe2x80x9ccentral center-cut holexe2x80x9d denotes a single loaded blast hole at the center of the center-cut region. xe2x80x9cAuxiliary center-cut holesxe2x80x9d denote the center-cut holes except for the central center-cut hole arranged around the central center-cut hole in a circle having a predetermined diameter. The auxiliary center-cut holes are comprised of unloaded and loaded auxiliary (blast) holes. xe2x80x9cSpreader center-cut holesxe2x80x9d denote loaded holes that are arranged in circles around the auxiliary center-cut holes.
As illustrated in FIGS. 2a and 2b, a conventional tunnel blasting is performed, in such a way that center-cut blast is primarily performed using a cylinder-cut method to obtain dual free surfaces, and the spreader center-cut holes, the cut spreader holes 2, the floor holes 3 and the roof holes 4 are secondly and sequentially blasted.
That is, the conventional tunnel blasting is the blasting in which the spreader center-cut holes, the cut spreader holes 2, the floor holes 3 and the roof holes 4 are sequentially blasted after center-cut blasting is performed by blasting the loaded holes (center-cut holes) each having a diameter of 38 to 45 mm drilled around one to four unloaded holes each having a diameter of 65 to 120 mm.
In the conventional tunnel blasting employing the cylinder-cut method, when center-cut blasting is not sufficiently performed, dual free surfaces are not easily formed during the blasting of the other blast holes, thereby causing many remaining holes to exist. Accordingly, the excavation efficiency is relatively low, that is, about 90% of a drill footage (less than 80%, depending upon the quality of rock). As a result, the advance formed by a single set of blasting is relatively short, so that the number of sets of blasting should be increased.
Because of the problem, there occur problems in which the work period cannot be shortened owing to a long excavation period and a long reinforcement period, excavating costs and drilling costs per blasting set are excessively high, and divisional blasting should be performed to minimize blast vibrations.
In the meantime, conventional center-cut blasting is performed by slant hole center-cut blasting (that is, V-cut blasting) or horizontal center-cut blasting (that is, cylinder-cut blasting; the cylinder-cut blasting is an improvement from burn-cut blasting).
The V-cut blasting is applied where slant center-cut holes are short, outside holes (such as cut spreader holes, floor holes and roof holes) are long, and the drill footages and drill angles of blast holes can vary. The V-cut blasting is chiefly applied to the blasting of short holes in which its advance is less than 2 m. The cylinder-cut blasting is applied where one to four unloaded holes each having a diameter shorter than the diameter of each loaded hole are drilled in parallel with the tunnel axis to the same drill depth as that of the loaded hole. The cylinder-cut blasting is chiefly applied to the blasting of long holes in which its advance is longer than 2 m.
As illustrated in FIGS. 3a and 3b, the V-cut blasting is performed in such a way that three or four sets of loaded central holes are drilled in the central region of a tunnel in parallel with one another with each set comprised of two opposite loaded central holes, the loaded central holes are simultaneously blasted to create a new free surface, and outer holes are blasted in the order of spreader center-cut holes, cut spreader holes, floor holes and the roof holes to expand the created free surface.
The V-cut blasting is center-cut blasting that has been employed for the longest time. In the V-cut blasting, the bottoms of the drilled center-cut holes are situated in a line with two center-cut holes of each set facing each other, and the interval between two burdens is 100 to 150 mm. Accordingly, the volume of a fractured rock portion is large and the projected area of blasting is wide due to the slant center-cut holes, large fragments are easily formed during center-cut blasting, and the center-cut holes can be drilled in various patterns.
The V-cut blasting is advantageous in that in comparison with the burn-cut and cylinder-cut blasting, the drilling of holes is easy, the drilling footages of holes are short, the flying distance of fracture is short owing to the creation of large fragments, the V-cut blasting is effectively applied to the blasting of short holes or a soft rock, the occurrence of dead pressure may be generated, and a target drill footage can be achieved regardless of inferior drilling due to a large free surface.
However, the V-cut blasting is disadvantageous in that its advance is restricted, a plastic region and extra excavation are increased due to its blast vibrations, the actual drill footages of the holes are short and blasting efficiency is low owing to slant drilling, secondary blasting is required due to the creating of large fragments, and the V-cut blasting is improper for precision rock blasting because of its great vibrations.
In addition, accidents due to falling rocks fall may occur due to the remaining holes, a planed excavation time is lengthened due to the delay of floating rock removal time and next drilling time, and the sectional area of a blasted tunnel is limited owing to slant drilling.
Furthermore, the V-cut blasting has defects in that blasting failure may occur due to the drilling error of V-shaped holes, large fragments may be created due to concentrative loading, and the creation of free surfaces is not easy. The V-cut blasting has a mechanism in which the V-shaped holes are initially blasted and thereafter the other holes are sequentially blasted.
On the other hand, as depicted in FIGS. 4a and 4b, the horizontal center-cut blasting is applied to the blasting of long holes. Burn-cut blasting and cylinder-cut blasting are generally employed for the horizontal center-cut blasting. In the burn-cut blasting, a plurality of unloaded holes each having the same diameter as that of each loaded hole are drilled. In the cylinder-cut blasting, one to four unloaded holes each having a diameter greater than that of each loaded hole, for example, 65 to 120 mm, are drilled.
In those horizontal center-cut blasting methods, drilled but unloaded holes serve as auxiliary free surfaces (small free surfaces) during the blasting of the loaded holes, so a center-cut operation is facilitated. A free surface F formed after the blasting of the center-cut holes is sequentially expanded to the cut spreader holes, floor holes and roof holes in order.
Accordingly, center-cut holes are drilled perpendicular to the free surface F and in parallel with one another, so that long holes can be drilled and thereby the drill footage each time is longer. The interval between a loaded hole and an unloaded hole is different depending upon the property of the explosive used and the quality of the rock, but generally 10 to 30 cm. The center-cut holes are blasted in a concentrative blasting fashion, or using precise delay detonators.
The horizontal center-cut blasting is advantageous in that its blast vibrations are weak in comparison with the V-cut blasting, the sectional area of a tunnel is not restricted due to horizontal drilling, dead pressure is not generated, and transportation and storage efficiency is superior due to the uniformly sized fractures.
On the other hand, the horizontal center-cut blasting is disadvantageous in that the charge applied to surrounding holes around a burn-hole is large in the case of burn-cut blasting, and blast vibrations may be increased when unloaded holes do not serve as free surfaces due to their small diameters.
In addition, a bit and a rod should be replaced with new ones in order to drill large holes, the remaining holes exist, advanced drilling technique is required to prevent blasting efficiency due to a drilling error, and the flying distance of the smallest fragment is long.
Further, the charge applied to center-cut region is large due to the small fractured volume of the center-cut holes, a working face and surrounding rock portions are damaged, and drilling time is lengthened due to the drilling of large unloaded holes and a plurality of loaded holes around the large unloaded holes.
In the conventional center-cut blasting methods, explosives of the quantity greater than a standard the charge is loaded in the blast holes so as to achieve complete free surfaces in a center-cut region, so that strong vibrations occur owing to the excessive charge per delay, thereby allowing the strong vibrations to damage a mother rock.
Additionally, when the V-cut blasting or cylinder-cut blasting is applied to the center-cut blasting, the remaining holes each having a depth corresponding to 10 to 20% of a drilling footage exist in the center-cut region and the sounding region. In this case, the next blasting should be delayed, so that excavation efficiency is lowered.
In particular, since the considerable portion of a tunnel is blasted in a hard rock region corresponding to Π or  degrees on the basis on the rock mass rating, there is investigated a scheme for improving excavation efficiency by the increase in the advance per blasting set.
In order to overcome the problems of the conventional center-cut blasting methods, a center-cut blasting method for tunnel excavation is developed by introducing pre-splitting technique and sub-drilling technique. In accordance with the pre-splitting technique, a circular pre-split is created by drilling large unloaded holes and loaded holes in a circle and blasting the loaded holes utilizing a detonating fuse and precision explosives, so as to facilitate the achievement of free surfaces. In accordance with the sub-drilling technique, the center-cut holes are drilled additionally but outsides holes except for center-cut holes are not drilled additionally, so as to maximize the advance per blasting set, minimizing the remaining holes and eliminating the sub-drilling of outside holes.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a center-cut blasting method for tunnel excavation utilizing large unloaded blast holes and pre-splitting, in which the binding force of an initial rock is weakened by utilizing large unloaded blast holes and pre-splitting so as to easily form dual free surfaces, thereby facilitating center-cut blasting and thereby improving the efficiency of tunnel blasting.
Another object of the present invention is to provide a center-cut blasting method for tunnel excavation utilizing large unloaded blast holes and pre-splitting in which loaded blast holes and unloaded blast holes are additionally drilled to achieve the maximum advance, thereby shortening the period of tunnel excavation and improving the economical efficiency of tunnel excavation.
In order to accomplish the above object, the present invention provides a center-cut blasting method for tunnel excavation, comprising the steps of: drilling a single central center-cut hole at the center of a center-cut region, drilling a plurality of auxiliary center-cut holes comprised of large unloaded auxiliary holes and loaded auxiliary holes that are alternately arranged around the central center-cut hole to be situated in a circle having a predetermined diameter, and drilling a plurality of spreader center-cut holes outside the auxiliary center-cut holes to be situated in concentric circles centered by the central center-cut hole; loading the center-cut holes with delay detonators and explosives and stemming the center-cut holes with stemming material at their entrances; blasting the loaded auxiliary holes of the auxiliary center-cut holes so as to create a circular pre-split; blasting the central center-cut hole so as to create initial dual free surfaces; and sequentially blasting the spreader center-cut holes with a time delay so as to create final dual free surfaces.